1. Technical Field
The present invention relates to measuring the moisture content of heavy grades of paper (for purposes herein, paper having a fiber weight of at least 300 grams per square meter) during continuous manufacture. More particularly, the invention relates to apparatus and methods which can be used to effect such measurement by infrared absorption means.
2. Discussion of Related Art
The art of measuring the moisture (water) content of paper during its continuous manufacture by using the correspondence between the absorptance of particular infrared radiation wavelengths and the relative amounts of particular components present in the paper has been practiced for many years. Early approaches that were commercially successful are exemplified by U.S. Pat. No. 3,405,268 Brunton. Modified forms of these early approaches continue to be used.
In its simple form, the measurement is made by determining transmittances through the paper for two narrow bands of infrared radiation, one of which is very sensitive to absorption by water, (the moisture "absorption" or "measurement" band), and the other of which is relatively insensitive to absorption by water (the moisture "reference" band). The ratio of these transmittances is a function of water weight per unit area. The moisture measurement may be further refined by similarly measuring the cellulose fiber content of the paper and correcting the moisture measurement for scattering or absortion effects produced by the presence of fiber or other components. (See, e.g. U.S. Pat. Nos. 3,551,678 Mitchell and 4,577,104 Sturm.)
The successful use of infrared radiation for moisture measurement of paper has been limited to light or medium grades. In past efforts to address this limitation, it has been recognized that the limitation results from the very low penetrability of heavier grades by energy associated with conventional measurement and reference bands of both fiber and water. These problems are discussed in U.S. Pat. Nos. 3,793,524 Howarth and 3,551,678 Mitchell. The Mitchell patent suggests that the limitation can be "eased" by use of the O-H stretch region centered at about 1.45 microns in a case where a very high moisture content prevents transmission of measurable amounts of energy associated with the moisture absorption band. (Ordinarily, bands having wavelength centra of about 1.92 to about 1.96 microns are used for the moisture absorption band.). However, the use of a moisture absorption band centered at 1.45 microns is generally impractical because of the relatively high absorptances for both moisture and cellulose fiber at that wavelength. Furthermore, it appears that the primary problem in determining the transmittances of moisture absorption bands is not the inability to penetrate the paper with sufficient radiant energy, but rather the inability to produce precise determinations of these transmittances under conditions of varying temperature. This imprecision results from variations in the transmission spectrum for moisture with changes in temperature. Past methods aimed at compensating for this variation have employed thermistors or pyrometers. These attempts have not succeeded with heavy grades, possibly because they measure the surface temperature of the paper rather its average temperature throughout the entire thickness dimension.
Accordingly, contemporary approaches to measuring the moisture content of heavy grades of paper employ different means based on different physical phenomena. A favored approach has been to use microwave moisture gauging techniques. However, in addition to their considerable expense, these gauges perform poorly on paper which has a low fractional moisture content wherein bonding between water molecules and cellulose inhibits molecular rotation.
It has been discovered that very satisfactory measurements of the moisture content of heavy grades of paper can be made by infrared absorption means. It appears that these measurements, not being influenced by water/cellulose bonding, are superior to those produced by microwave moisture gauges.
An object of this invention is to provide apparatus and methods that enable measurement of the fractional moisture content, or percent moisture, of heavy grades of paper by infrared absoption means.
A further object of the invention is to provide apparatus and methods that overcome prior limitations to such measurement that arise from inadequate penetration through the paper by radiant energy associated with conventionally-employed fiber absorption bands.
Another object of the invention is to provide apparatus and methods for calculating the average temperature of a moving sheet of heavy-grade paper.
Another object of the invention is to provide apparatus and methods that produce accurate indications of the moisture weight per unit area of a sheet of paper, which when combined with conventionally-determined indications of basis weight or fiber weight, enable a measurement of the fractional moisture content of the paper.
Yet another object of the invention is to provide apparatus and methods as described which can be adapted to measure the moisture content of light or medium grades of paper.